Mechanisms for nutrient delivery to the inner shelf: observations from the Santa Barbara Channel

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Mechanisms for nutrient delivery to the inner
shelf observations from the Santa Barbara Channel
Erika McPhee-Shaw March 12, 2003 David
Siegel Libe Washburn Mark Brzezinski -
UCSB Dan Reed - UCSB Dick Zimmerman - Moss
Landing David Salazar, Janice Jones, Mike
Anghera, Chris Gottschalk, Leah Ow, Bryn Evans,
Helene Scalliet
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• Santa Barbara Channel LTER (Long-Term Ecological
  Research)
• NSF-funded LTER Long-term
  interdisciplinary study of an ecosystem
  (giant kelp forests) at the land/ocean
  interface.
• General LTER Objectives
• study spatial and temporal scales of
  terrestrial and oceanographic forcing
• determine the relative importance of
  terrestrial versus oceanic sources of
  nutrients and other constituents to kelp
  forests
• SBC is characterized by an abrupt transition to
  low surface nutrient concentrations (south of
  Point Conception), so summer nitrate
  limitation can be important.

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Outline
1. Description of nearshore observations A.
Identifying mechanisms for nitrate supply B.
Nitrate supply budget (addressing first LTER
question) 2. Starting to examine the dynamics of
cross-shelf exchange A. High frequency
(diurnal) events B. Low frequency
events What is going on? Work in progress.
Focus on mechanisms for nutrient delivery to kelp
reefs Spring upwelling Internal
waves Eastward advection/episodic flow
reversals Terrestrial input
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• MEASUREMENTS
• Time series from near-shore stations, Feb
  2001 Aug 2002
• Three Stations 10, 15, 17 m water depth
• Continuous data
• - CTD, optical backscatter, fluorescence at 4
  m depth
• - bottom-mounted ADCP
• - Thermistors at three depths
• Nitrate auto-analyzer (W.S. Oceans Wet
  Chemical Analyzer)
• - Six separate deployments (covering 23 of
  one year)
• - (moored at different nearshore locations and
  depths)
• One year nutrient supply budget July to
  July 2001- 2002

Additional Data - CTD/water chemistry at
nearshore sites, monthly - CODAR
(high-frequency radar) - surface currents -
Cross-channel hydrography, 3 LTER cruises per year
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Santa Barbara Channel
Satellite SST image courtesy of Libe Washburn
Upwelling common north of Point Conception
Within channel, a cyclonic synoptic state most
common Hendershott and Winant, 1996
- Harms and Winant, 1998
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Santa Barbara Channel near-shore stations
Sources of Nutrients SPATIAL GRADIENTS Vertical
gradient nutricline depth 30 to 50 m in
summer 0 to 10 m in winter
Geographic gradient Cold water, higher
nutrients, north of Point Conception (where
upwelling is common)
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Vertical Gradients Nutrients at depth.
Depth of nutricline changes
seasonally.
oC
mmol/L
From Plumes and Blooms mid-channel station -
(plot from Olga Polyakov)
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Feb, 2001 to July, 2002
Winter
Upwelling
Summer
Upwelling
Summer
Naples Mooring
T
S
U alongshore velocity (East positive)
Carpinteria Mooring
One big storm event
T
S
U alongshore velocity (East positive)
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Measured Nitrate, July 2001 to July 2002
Temp
Measured NO3
When Nitrate measurements not available, use
temperature as a proxy
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Estimated nitrate over one year, July 2001 to
July 2002
Temp
N estimated from near-surface temp (blue) In-situ
N (green)
mmol/l
N estimated from near-bottom temp (blue) Measured
N (green)
mmol/l
upwelling
internal waves
summer
advection
winter
Nitrate delivery mechanisms Upwelling Internal
waves Advection
Background States Winter NO3 1.5 to 2
mmol/L Summer NO3 0.1 to 0.5 mmol/L
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December, 2001, Advection from western channel -
associated with flow reversal
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Spring Upwelling Event April to May, 2002.
Temperature
Measured NO3
U along-isobath
V cross-isobath
m/s N
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Upwelling in the Santa Barbara Channel no
simple relationship with local winds
March - April 2001
Temperature
??
N-S Wind
E-W Wind
March 2002
Temperature
N-S Wind
E-W Wind
(Winds from Pt. Arguello station)
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Summer high stratification, diurnal
oscillations in temperature and cross-slope flow
Naples, July 2002, no NAS yet
temperature depths 3m, 4 m, 9m, 14m
m/s onshore
Temperature at 3 different depths,
Arroyo Quemado, July 2002 with NAS deployment
Nitrate measured with NAS
North-south velocity component
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Nitrate deployments - estimating nitrate supply
rate from different mechanisms
N(t) No/T U N mmol/L/ day
Supply Mechanisms
Upwelling 0.69
(mmol/l/day)
Advection from west 0.44
Internal waves Summer 12- m depth
0.07
"Background Conditions" (no identified advective
fluxes, background sources approximately balance
sinks)
0.03
Winter
0.01
Summer (surface)
One storm event from previous year - March 2001
- N estimated from freshwater dilution
and NO3 measured in local streams
0.13
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Annual Nitrate Delivery Budget July 2001 to
July 2002
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Nitrate Budget - general mechanisms identified.
Now what? ---------------------------------------
--------------------------------------------------
-------------------- 1. Modeling kelp uptake as
function of Nitrate(time) ? Biomass(time) (with
Dick Zimmerman, will compare to kelp growth rate
data collected by Dan Reed) 2. Understanding
dynamics of cross-shelf exchange. Communication
to inner shelf Nearshore often
neglected Diurnal / tidal-frequency
dynamics Low-frequency dynamics (upwelling,
flow reversal events)
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Tidal-frequency / diurnal internal motions
Summer baroclinic oscillation diurnal signal
dominates
Barotropic tides mixed semidiurnal and diurnal
1 cpd
Naples, Summer 2001.

• Common feature in Southern California Bight
  (Winant and Bratkovich, 1981, Zimmerman and
  Kremer, 1984, Pineda, 1995, Lerczak et al., 2001,
  Boehm et al. 2003, Hamilton and Noble, 2003)
• Important for transport of sewage to surf zone,
  Huntington Beach (Boehm et al, 2003)
• Summertime, stratified conditions. Nutrient
  limitation important for kelp ecosystems in
  SCB. Internal waves only source delivering deep
  water to inner shelf. (Zimmerman and Kremer,
  1984, Kopczak et al., 1991)

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• Forcing?
• Frequency is sub-inertial (f 1.13 cpd at 34o
  lat). Free propagation of internal waves not
  allowed at 1 cpd.
• Suggested mechanisms
• 1. Generation by tides? Pineda, 1995, suggests
  relationship to spring/neap cycle
• 2. Diurnal Winds/ Daily sea breeze.
• Sea breeze relatively strong in S. CA. Bight
  where coastal winds weak
• Lerczak et al, 2001. Suggest surface forcing by
  diurnal winds, coupled with changes to relative
  vorticity (lowering effective f) due to mesoscale
  eddies, cross-basin shear of geostrophic
  currents.
• Santa Barbara Channel?
• Observed diurnal perturbations not associated
  with phase of tide or spring/neap cycles
  (Cudaback, UCSB)
• Diurnal frequency dominant in wind. However,
  gyre in the channel is almost always cyclonic ?
  positive vorticity.

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Does inner-shelf respond to remote forcing?
High correlation between 15-m temperatures at San
Diego, Huntington Beach, and Santa Barbara
suggest yes. (Jamie Pringle, UNH) Temperatures
best explained by remote winds, hundreds of km
to South (coast of Mexico) Time lag
appropriate for phase speed of coastal trapped
wave
Figure from Jamie Pringle. Data courtesy of the
Pt. Loma Kelp Forest Project, Orange County
Sanitation District, and the Santa Barbara LTER.

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Are episodes of diurnal internal wave activity
associated with remotely-forced, low-frequency
changes in temperature or density structure?
Diurnal temperature fluctuations
Band-passed diurnal variance
Band-passed temperature
Correlation R 0.41, max at lag of 5 days
between variance and temperature (i.e. events
associated with low-frequency cooling
Maybe? . . . . . . .
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Low-frequency events - Spring upwelling, flow
reversals
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• High Wind Stress Curl near Point Conception
• ? Strong Ekman pumping (Munchow, 2001)
• This mode accounts for 72.5 of variance in
  spatial wind field (Munchow, 2001)

• What happens when wind stress curl episodically
  dies off? (other 27.5)??

Pressure gradient possibilities that could drive
equatorward flow along coast
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"Typical Conditions"
Cross-channel Scanfish (towed CTD) sections -
Potential density September, 2001
Lines A and B
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12 to 24 hours before intense cooling is observed
at the coast
Cross-channel Scanfish (towed CTD) sections
Potential density April, 2002
Lines A and B
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Low-frequency nearshore cooling events "Spring
upwelling" and "episodic reversal" events may be
caused by similar dynamics Seasonal Context
Both occur during seasonal transition
Summer Strong stratification July -Sep 2001
Winter Weak stratification Jan-April 2001
Temperature at 40-m depth
s? at 40-m depth
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Conclusions Nitrate Budget for inner shelf (reef
depths of 10 to 15 m) Low-frequency events such
as spring upwelling and episodic flow reversals
are dominant suppliers of nitrate to inner
shelf Internal waves are important in summer,
when nutrients are limiting at shallow, kelp reef
depths Terrestrial runoff events have not been
important during observational period Mechanisms
for cross-shelf transport We need to better
understand the link between inner-shelf dynamics
and those of the outer shelf and
basin Implications for the rest of the S. CA.
Bight? Other coastal regions?
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EXTRAS FOLLOW
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Diurnal Tidal Oscillations temperature
fluctuations vary with depth onshore /
offshore current reversal with depth
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NAPLES. Summer 2002 - Jun 22 to July 22, 2002
Vertical Mode structure from EOF on the complex
velocity vector. Baroclinic, (mode-1) signal has
primary variance along an axis 22 to 25o south
of east, which is about 35 to 40 off of the
barotropic velocity - May be important that it
is NOT exactly across-isobath, nor is it
perpendicular to main currents
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Wind Spectra
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High Coherence between nearshore stations at
diurnal frequency
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NAPLES Jun 15 to July 15, 2002
Baroclinic currents
Barotropic currents
Arroyo Quemado - Jun 24 to July 10, 2002
(Nitrate deployment)
Measured Nitrate
Baroclinic currents
Barotropic currents
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Local Winds and Local (Springtime) Upwelling ??
----- Complicated
N
E
E
N
E
E
N
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Uptake modeling - SIMPLISTIC APPROACH Assume
a constant tissue N ratio of 3 dry weight. Make
a direct jump from nitrate uptake to biomass
growth rate. N(t) No ?dN/dt dt , dN/dt
nitrogen uptake rate N(t).(Umax) NO3/(HN
NO3). Values for these parameters, according
to R. Zimmerman, are the following HN
(half-saturation Nitrogen) 7 to 10 uMol Umax
0.5 to 1 uMol N per hour per fresh-weight gram
of kelp Initial biomass from LTER-measured
kelp frond density of 11 fronds m-2, ----- do
some scary unit conversions and estimate dry
weight 20 fresh weight,
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Measured nitrate and estimated N uptake rate
January 2002
April-May 2002
June, 2002
Ks 7 uM
Estimated kelp growth rate, compared to 1 per
day growth (black line)
January 2002
April-May 2002
June, 2002
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Uptake and growth modeled for 2001 NAS deployments
October, 2001
December, 2001
Nitrate blue Uptake rate -green
October, 2001
Black lines represent 1 per day growth rate
December, 2001
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(AVHRR composites from Mark Otero, UCSB)
Upwelling common north of Point
Conception Within channel, a cyclonic synoptic
state most common Hendershott and Winant, 1996
- Harms and
Winant, 1998
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Winds at West - Channel buoy, 2001
upwelling period
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Upwelling in the Santa Barbara Channel no
simple relationship with local winds
March - April 2001
Temperature
??
N-S Wind
E-W Wind
March 2002
Temperature
N-S Wind
E-W Wind
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Advection Events sustained eastward flow is rare
advection_upwelling.m
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This is how I approached this problem. These
models are simply modeling nitrate uptake and
resulting increase in kelp biomass assuming a
constant tissue N ratio of 3 dry weight. By
using a constant ratio, we can leap from nitrate
uptake directly to biomass growth rate. Eqn for
nitrate N(t) No ?dN/dt dt , where the dN/dt
term is described by a nitrogen uptake rate,
which follows Michaelis-Menten dynamics, and is
proportional to 1) the biomass, and 2) Umax
NO3/HN NO3. Values for these parameters,
according to R. Zimmerman, are the following HN
(half-saturation Nitrogen) 7 to 10 uMol Umax
0.5 to 1 uMol N per hour per fresh-weight gram of
kelp. I.e., this has to be multiplied by the
(concentration of) grams fresh weight of kelp
biomass. Eek. This requires some thinking, since
you have to use dry weight and percent tissues
nitrogen to relate this uptake of N back to
biomass, and yet it is expressed as a rate PER
GRAM of WET KELP WEIGHT. So, at each time step
you have to know that as well. I decided to leave
it per biomass units of grams fresh, but did
convert it to day-1, since everything else Ive
done until now was in days. NO3 input
nitrogen in water observed from NAS. Initial
Values Biomass and N Need to get concentration
of biomass in two sets of units First) grams
fresh weight for calculating the correct total
uptake rate. Second) grams of dry weight
converted to nitrogen mass in uMol. For correctly
integrating the nitrogen uptake rate with time to
an integrated mass of N within the kelp, then
getting from that back to the kelp biomass (info
from Dick from Dans measurements). Kelp frond
density 11 fronds m-2 Kelp frond mass 1 kg
frond-1. 1) Initial Kelp biomass density (in
grams, nominally per kg water to match above
concentrations in uMol uMol/L) 11
kg/m2(1000g/kg) (1/1m) 11 grams. (this was
assuming that the biomass was all in the top 1
meter, so the density of 11 fronds m-2 is 11
fronds m-3. 2) Initial N in biomass Assume
dry weight 20 fresh weight. Assume tissue N
concentration 3 Np_o 0.20.0311 grams14E3
uMol/gram 924 uMol N. Solutions to biomass as
a function of time are shown in the following
graphs, for the observed nitrate concentrations
weve observed with the NAS.
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Santa Barbara Channel Synoptic States
AVHRR composites from Mark Otero, UCSB
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Terrestrial Runoff Events
Hydrograph Winter Spring, 2001. Santa Barbara
Streams USGS gauges Mission Creek at Rocky Nook
and Mission St. Atascadero at Patterson San Jose
at Goleta
(data from Ed Beighly)
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March Storm event
(40-day period)
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Nutrient Concentrations in Santa Barbara
Streams, Storm of March 3 6, 2001.
Mean Nitrate Concentrations (mM/l) during the Mar
3 -6 storm Arroyo Burro 71.3 Atascadero 64.
6 Mission 52.2 Franklin 297 (?) Santa
Monica 50.4 Carpinteria 44.0 Rincon 55.6 Arroyo
Hondo 61.9 ------------------------------------- m
ean (without Franklin) 57 mM/L mean
(including Franklin) 87 mM/L
(From Lydecker data report, 2001)
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Use dilution by freshwater to estimate NO3
concentration Maximum freshwater dilution was
7.6 Seawater initial NO3 1
mM/L Freshwater initial NO3 40, 60 80, 100
mM/L
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Maximum NO3 concentration estimates from storm
event (for freshwater range of 40 to 80
mM/L) 3.8 to 8.3 mM/L at Carpinteria 3.1 to
6.3 mM/L at Naples
Ocean measurements from March 7, 2001
Carpinteria Offshore 1 m - 5.6 mM
5 m - 4.1 10 m - 2.4 Reef 1 m -
5.1 5 m - 3.6 Inshore 1 m -
5.5
Naples Offshore 1 m - 3.0 mM 5 m -
2.2 10 m - 2.1 Reef 1 m - 1.9
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WARM
Advection back and forth across shelf via
internal tides
COLD
Implications for nutrient delivery?
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